Abstract

Co-firing biomass with coal at existing power plant is widely adopted as one of the main technologies for reducing CO2 emissions in the UK and the rest of the world. Despite various advances in developing the co-firing technology, a range of technological issues remain to be resolved due to the inherent differences in the physical and combustion properties between biomass and coal. Typical problems associated with co-firing include poor flame stability, low thermal efficiency, and slagging and fouling. This project aims to achieve the optimisation of biomass/coal co-firing processes through a combination of advanced fuel characterisation, integrated measurement and computational modelling. In the area of fuel characterisation, both thermo-gravimetric analysis and automated image analysis techniques in conjunction with conventional fuel analysis methods will be combined to achieve comprehensive characterisation of biomass and biomass/coal blends from a wide range of sources. Because of the physical differences between biomass and coal the fluid dynamics of the biomass/coal/air three-phase flow in the fuel lines feeding the burners is rather complex and very little is known in this area of science. It is proposed in this project to develop an instrumentation technology capable of measuring the basic parameters of the biomass/coal particles in the fuel lines on an on-line continuous basis. The system will allow the monitoring and optimisation of the fuel delivery to the burners. The instrumentation technology combines novel electrostatic sensing and digital imaging principles and embedded system design methodology. The flow parameters to be measured include particle size distribution, velocity and concentration of biomass/coal particles as well as biomass proportion in the blend. It is known that biomass addition and variations in coal diet can have a significant impact on combustion stability and co-firing efficiency. As part of this project, a system incorporating digital imaging devices and solid state optical detectors will be developed for the continuous monitoring of the burner conditions and flame stability under co-firing conditions. Computational modelling provides a powerful supplementary tool to experimental measurement in the studies of three-phase flow and combustion flame characteristics. Computational Fluid Dynamic (CFD) modelling techniques will be applied in this project to investigate the dynamic behaviours of irregular biomass particles and their blends with pulverised coal in the fuel lines and associated combustion characteristics particularly flame stability. CFD modelling techniques will also be applied to study the impact of biomass addition on ash deposition and formation of slagging and fouling. The measurements from the flow metering and flame monitoring systems will be integrated to establish and validate the CFD models. Meanwhile, the modelling results will be used to interpret the practical measurements under a wide range of conditions.The project consortium comprises three academic centres of expertise including Kent, Leeds and Nottingham. Collaborative arrangements with three leading research centres in China have been established in addition to support from power generation organizations in the UK and China. Following the design and implementation of the instrumentation systems and computational modeling work, experimental work will be performed on combustion test rigs in both countries. The instrumentation systems and computational models will then be scaled up for full scale power stations. Demonstration trials will be undertaken to assess the efficacy of the advanced fuel characterisation techniques, the performance and operability of the instrumentation systems, and the validity of the computational models under a range of co-firing conditions. Recommendations for the optimization of co-firing processes at existing power plant and on the design of new plant will be reported.

1. As a result of this project a new instrumentation technology has been developed for the in-line continuous measurement of the basic parameters of the biomass/coal particles in the fuel pipes. The instrumentation technology combines novel electrostatic sensor arrays, piezoelectric sensors, digital imaging devices and associated digital signal and image processing algorithms. Prototype systems operating on the technology were developed and evaluated on to measure a range of flow parameters including particle size distribution, particle shape distribution, velocity and concentration of biomass and coal particles. Trials were undertaken on industrial-scale combustion facilities in the UK and China.

2. A new flame monitoring technology, incorporating imaging devices, solid-state optical detectors, image fibre bundles and optical spectroscopic components, have been developed for the continuous monitoring of flame temperature distribution, oscillation frequency and radiative profiles of free radicals as well as flame stability. Prototype systems based this technology has been evaluated on full-scale coal, biomass and heavy oil fired power stations in the UK, China and Saudi Arabia.

3. We have found that adding biomass to pulverised coal at existing coal fired power stations can have significant impact on the operation of the power plant in terms of fuel handling and flame stability. The presence of large and irregular biomass particles in the fuel affects the flow characteristics in the fuel injection pipeline and, more significantly, the stability and properties of the flame.

4. Significant further research is required to study the characteristics of biomass flow and biomass fired flames as many coal to coal/biomass fired power plants are being converted to 100% biomass combustion. Because of the physical differences between biomass and coal the fluid dynamics of the biomass/air two-phase flow in the fuel pipes is rather complex and very little is known.

Exploitation Route

The design of new instruments for pulverised fuel flow metering and advanced flame monitoring and associated experimental results have been published in a range of journals and presented at leading international conferences in the field. Three one-day workshops were held over the project period, two in China and one in the UK. The workshops were attended by both academics and industrialists across the UK and China. The results have helped the industrial partners, in particular, Drax Power, RWE npower, E.ON, Alstom Power and China Datang Corporation, to optimise their coal and biomass fired power plants, leading to improved plant efficiency and reduced pollutant emissions. The findings are also beneficial to academic researchers and practitioners working in particle technology, fluid mechanics, multiphase flow, material sciences, computational modelling, sensors and measurement sciences. The technology has been used by several coal fired power stations in China.